Anatomy of the Food Mollusks 21 



the sides of the body, and the body wall itself, have been 

 removed so as to expose the digestive tract and other in- 

 ternal organs. In most animals that possess a tubular 

 digestive tract, the mouth is to be found at the forward 

 or anterior end of the body, and the rule holds in this case. 

 Usually, also, this opening into the digestive tract is 

 situated in a modified part of the body that may be called 

 a head. This was probably true of the very early an- 

 cestors of the bivalves, but as the result of the develop- 

 ment of a shell completely covering the body, the head of 

 all living bivalves has disappeared as a distinctly modi- 

 fied region. For this reason they are sometimes called 

 Acephala. 



The mouth is not shown in either of the illustrations, 

 but its position in the oyster is indicated. The opening is 

 hidden by a pair of huge folds or lips, one placed in front 

 of, and the other behind it. These labial palps extend, 

 right and left, far out from the mouth, and are shown in 

 both figures. In Figure 3, the front or anterior palp on 

 the right side has been partially removed, and the inner 

 surface (that nearest the mouth) of the inner palp is 

 exposed. The organs are so situated that they may come 

 in contact with the anterior margins of the gills. The 

 latter are collectors of the microscopic food, which they 

 pass forward, by ciliary action cilia being minute hairs 

 that cover various surfaces and have a rapid lashing 

 movement to the inner surfaces of the palps. Over 

 these, in turn, it may continue forward to the mouth. 

 As will be shown later, when material is too abundant on 

 the palps, it is not directed to the mouth, but to tracts 

 that carry it out of the body. 



The mouth, having the form of a funnel, leads directly 

 into the oesophagus. This tube may be traced backward 



